1. Field of the Invention
The present invention relates to a so-called rectifier circuit converting an AC voltage to a DC voltage, and particularly to a rectifier circuit preventing a switching device from being damaged by an excessive inrush current flowing in the switching device at recovery of an AC power supply from an interruption or at turning-on the power.
2. Background Art
Previously, a rectifier circuit as shown in FIG. 19 has been known. In the rectifier circuit, fast recovery diodes D1 and D2 forming upper arms are connected to their respective switching devices Q1 and Q2 of MOSFETs forming lower arms in series to form legs (series circuits) connected in parallel. To the connection point of the fast recovery diode D1 and the MOSFET Q1 and the connection point of the fast recovery diode D2 and the MOSFET Q2, an AC voltage is supplied from an AC power supply 1 through inductors L1 and L2, respectively. Moreover, in parallel to the series circuits, a capacitor C for smoothing a rectified DC voltage Ed is connected so that the rectified DC voltage Ed is supplied to a load side.
An operation of thus formed rectifier circuit will be explained in order. The MOSFET Q1 is made turned-on when the polarity of a voltage Vin of the AC power supply 1 is positive (when the head of an arrow is on the positive side and the tail of the arrow is on the negative side, the same is true in the following). Then, a current increases while flowing along a path of the AC power supply 1→the inductor L1→the MOSFET Q1→a body diode of the MOSFET Q2 (illustrated with a broken line)→inductor L2→the AC power supply 1.
Subsequent to this, the MOSFET Q1 is made turned-off. Then, a current flowing in the inductor L1 and L2 gradually decreases while commutating along a path of the inductor L1→the fast recovery diode D1→the capacitor C→the body diode of the switching device Q2→the inductor L2→the AC power supply 1→the inductor L1. At this time, energy stored in L1 and L2 is supplied to the side.
Also in the case when the polarity of the voltage of the AC power supply 1 is negative (when the head of an arrow is on the negative side and the tail of the arrow is on the positive side, the same is true in the following), the symmetry of the circuit results in a similar operation by making the MOSFET Q2 turned-on and -off. With the MOSFETs Q1 and Q2 driven in this way by adequate control signals, while controlling the waveform of an inputted current sinusoidally, a desired DC voltage can be obtained. In the above-explained rectifier circuit, a step-up operation is carried out, by which a DC output voltage in a normal operation becomes equal to or more than the maximum value of the inputted AC voltage.
In the rectifier circuit formed as was explained in the foregoing, at turning-on the AC power or at recovery from a power interruption, the voltage of the AC power supply 1 sometimes becomes higher than the voltage across the capacitor C. At this time, with the polarity of the voltage Vin of the AC power supply 1 being positive, for example, an inrush current flows along a path of the AC power supply 1→the inductor L1→the fast recovery diode D1→the capacitor C→the body diode of the switching device Q2→the inductor L2→the AC power supply 1. That is, an excessive current is to flow in the fast recovery diodes D1 and D2 and the switching devices Q1 and Q2.
In a normal operation, when the switching device Q1 is made turned-on, the fast recovery diode D1 is made turned-off to apply the voltage across the capacitor C to the fast recovery diode D1. This brings the fast recovery diode D1 into a reverse recovery state. In the same way, when the switching device Q2 is made turned-on, the fast recovery diode D2 is brought into a reverse recovery state. This requires the use of fast recovery diodes each having a short reverse recovery time for the diodes D1 and D2. Each of a fast recovery diode and a switching device (including a body diode) has a low forward surge current capability, so that it might cause a possible damage when an inrush current flows. In addition, a Schottky diode of SiC (silicon carbide) as its material also has an excellent reverse recovery characteristic, so that it can be used as an alternative of each of the fast recovery diodes D1 and D2. Such a Schottky diode, however, similarly has a low current blocking capability that causes possible damage when an inrush current flows.
Thus, for protecting fast recovery diodes and switching devices from an inrush current, a rectifier system is known in which no excessive current is made to flow in such devices (see JP-A-2004-72846, for example). The rectifier system, as shown in FIG. 20, is provided with two legs (series circuits) formed of slow recovery diodes D10 and D11 forming their respective upper arms in the legs and thyristors Th1 and Th2 forming their respective lower arms in the legs. To the connection point of the AC power supply 1 and an inductor L1, the connection point of the upper arm with the slow recovery diode D10 and the lower arm with the thyristor Th1 in one series circuit is connected, and to the connection point of the AC power supply 1 and an inductor L2, the connection point of the upper arm with the slow recovery diode D11 and the lower arm with the thyristor Th2 in the other series circuit is connected. Moreover, the cathodes of the slow recovery diodes D10 and D11 are connected to on the positive polarity side of a DC power supply line and the anodes of the thyristors Th1 and Th2 are connected to on the negative polarity side of the DC power supply line.
With the rectifier circuits thus formed, the inrush current explained in the foregoing flows in the slow recovery diodes D10 and D11 and the thyristors Th1 and Th2 each having a large forward surge current capability without flowing in the fast recovery diodes D1 and D2 and the switching devices Q1 and Q2. Therefore, the rectifier system disclosed in JP-A-2004-72846 can be safely operated without damaging devices forming the system. The reference voltage of the controller of a rectifier system with such a configuration is an electric potential of the source of each of the switching devices Q1 and Q2. Compared with this, the thyristors Th1 and Th2 are driven with the electric potential of the cathode of each of them taken as a reference (see Denki Gakkai Handoutai Denryoku Henkan-houshiki Chousa Senmon Iinkai hen, Handoutai Denryoku Henkan Kairo, dai 1 pan, Shadan-houjin Denki Gakkai, 1987nen 2gatsu, p. 23, (Zu 2.5.2) (in Japanese), (IEEJ Semiconductor Power Conversion System Investigating Committee of Experts, Semiconductor Power Converter Circuit, 1st ed., p 23 (FIG. 2.5.2), The Institute of Electrical Engineer of Japan, February 1987), for example).
[Patent Document 1] JP-A-2004-72846
[Non-Patent Document 1] Denki Gakkai Handoutai Denryoku Henkan-houshiki Chousa Semmon Iinkai hen, Handoutai Denryoku Henkan Kairo, dai 1 pan, Shadan-houjin Denki Gakkai, 1987nen 2gatsu, p. 23, (Zu 2.5.2) (in Japanese), (IEEJ Semiconductor Power Conversion System Investigating Committee of Experts, Semiconductor Power Converter Circuit, 1st ed., p 23 (FIG. 2. 5. 2), The Institute of Electrical Engineer of Japan, February 1987)
The reference voltage of the controller of the above explained rectifier system is the electric potential of the source of each of the switching devices Q1 and Q2. Compared with this, the thyristors Th1 and Th2 must be driven with the electric potential of the cathode of each of them taken as a reference. Therefore, the driving circuit necessitates devices such as a pulse transformer for insulation as is shown in the “Semiconductor Power Converter Circuit” (non-patent document). This results in a complicated driving circuit in the above explained rectifier system, which causes a new problem in that the system becomes large-sized and highly expensive.